1. Field of the Invention
The present invention relates to sheet processing apparatuses and image forming systems, and in particular to a sheet processing apparatus that performs a folding process on a sheet-like recording medium conveyed (in the present specification, referred to as sheet) such as paper, transfer paper, printing paper, and an OHP transparency, and an image forming system that includes the sheet processing apparatus and an image forming apparatus such as a copier, a printer, a facsimile, and a digital multifunction peripheral.
2. Description of the Related Art
As a sheet processing apparatus that performs a folding process on a sheet conveyed from an image forming apparatus, a technology described in Japanese Laid-open Patent Publication No. 2006-117383 is known, for example. This technology features a sheet processing apparatus that includes a first stopping member the position of which provided in a second conveying path is movable to stop a leading end of a sheet, a conveying roller pair formed of a first conveying roller and a second conveying roller that nips the deflection of the sheet to form a crease at the first stopping member, a second stopping member the position of which provided in a first conveying path is movable to stop the sheet that passed the conveying roller pair, and a conveying roller pair formed of the second conveying roller and a third conveying roller that nips the deflection of the sheet to form a crease at the second stopping member, and forms a double parallel fold by controlling the stopping position of the second stopping member 10.
This technology is to perform what is called a leading-end abutment folding process by providing a dedicated path bifurcated from a conveying path, in which a sheet conveyed from an upstream device is conveyed to a downstream device, and stoppers to perform the folding process, and by abutting the leading end of the sheet on the stopper. More specifically, in the folding process, the folding position is adjusted and a deflection is formed by abutting the sheet on the stopper in the dedicated path, and the deflection formed is nipped by a folding unit to fold.
Meanwhile, the technology described in Japanese Laid-open Patent Publication No. 2007-277006, for example, is also known. This technology features a method of folding a medium by a folding device that includes a rotatable folding cylinder, a rotatable first press member that engages with the folding cylinder to form a first folding pinch, a rotatable second press member that engages with the folding cylinder to form a second folding pinch, and a media feeding unit, and the method includes feeding the medium by the feeding unit toward the cylinder that is intermediate between the first pinch and the second pinch, rotating the cylinder in a first direction to direct the medium in the first pinch to form looseness in the medium in the middle of the feeding unit and the cylinder, and rotating the cylinder in a second direction opposite to the first direction to move the looseness in the second pinch to fold.
More specifically, as the conventional methods to fold, two methods are generally used; one is to adjust the folding position, when a sheet on which an image is formed is received and a folding process such as letter fold and Z-fold is performed thereon, by abutting the leading end of the sheet on a sheet leading-end abutment member that is operable in accordance with the sheet size as disclosed in Japanese Laid-open Patent Publication No. 2006-117383 (hereinafter, referred to as a stopper method), and the other is to adjust the folding position by adjusting only the amount of conveyance by a conveying unit as disclosed in Japanese Laid-open Patent Publication No. 2007-277006 (hereinafter, referred to as a nip-reverse method).
In the stopper method described in Japanese Laid-open Patent Publication No. 2006-117383, the leading end of a folded portion of the sheet is abutted on the stopper, and in the second folding process, a sheet portion (a single sheet portion) on the first fold side, which is formed in the first folding process, is in a stopped state at all times. Consequently, the deflection of the sheet that arises in the second folding process is nipped by a second folding roller pair, and after the deflection is eliminated, the sheet portion (a single sheet portion) on the first fold side is then started to move, and thus a duplicate fold Pc is not likely to occur. However, a mechanism to move the sheet leading-end abutment member in accordance with the length of the sheet is necessary, and thus the downsizing of the apparatus is difficult as the installation space for the mechanism is essential.
Meanwhile, in the nip-reverse method described in Japanese Laid-open Patent Publication No. 2007-277006, the folding position is adjusted by only the adjustment of the amount of conveyance, and thus it excels in terms of downsizing. In letter fold, Z-fold, and such in which a sheet folding process is performed twice, however, when the second sheet folding process is performed, it is necessary to make an upstream side conveying unit (hereinafter, referred to as a first folding roller pair) and a downstream side conveying unit (hereinafter referred to as a forward-reverse roller pair) convey in directions conflicting with each other to form a deflection in the sheet. In this case, the first folding roller pair conveys the sheet in the downstream direction while the forward-reverse roller pair conveys the sheet in the upstream direction. Then, a second folding roller pair positioned downstream of the forward-reverse roller pair performs the second folding process on the deflected sheet. In such case, two creases referred to as duplicate folding or a duplicate fold may result.
FIGS. 28 to 31 are explanatory diagrams illustrating the mechanism of a duplicate fold to arise. In letter fold, Z-fold, and such in which the sheet folding process is performed twice, when the second sheet folding process is performed, a first folding roller pair 2 and a forward-reverse roller pair 3 are made to convey a sheet P in directions conflicting with each other (the first folding roller pair 2 in the downstream direction and the forward-reverse roller pair 3 in the upstream direction) to form a deflection Pt1 in the sheet P, and the second folding process is then performed by a second folding roller pair 4. Consequently, a sheet portion (a single sheet portion) P1 a including a leading end P1 of the sheet P on the first fold side, which is formed in the first folding process, may be drawn into the nip of the second folding roller pair 4 before a deflection Pt2 disappears (FIG. 29). When the second folding process is performed under this condition (FIG. 30), what is called a duplicate fold Pc arises in which two creases Pc1 and Pc2 are formed (FIG. 31).
To prevent this duplicate fold Pc from arising, when it is attempted to stop the leading end P1 of the sheet P formed in the first folding process (stop the forward-reverse roller pair 3) short of the nip of the second folding roller pair 4 (FIG. 32), the second folding roller pair 4 and the forward-reverse roller pair 3 are to pull the sheet P from both sides at an instant the deflection Pt2 in the second folding disappears (FIG. 33). Thus, it only needs to rotate the forward-reverse roller pair 3 at the instant the deflection Pt2 of the second fold disappears to eliminate the pull from both sides. However, it is not practical to configure the forward-reverse roller pair 3 to rotate in the foregoing manner to eliminate the pull from both sides in terms of dynamics and control due to inertia, and thus it is difficult to eliminate the sheet P to be pulled from both sides with such a mechanism.
There is a need to prevent a duplicate fold from arising in the second sheet folding process when the folding process is performed by the nip-reverse method.